Rubber products such as conveyor belts, hoses and rubber linings are frequently used in a harsh environment subject to strong vibration or stress. Polyolefin resins such as polyethylene resins and polypropylene resins have high rigidity and high weatherability as compared to rubber, and hence if the production of composite materials from polyolefin resins and rubbers is made possible, high performance of rubber products can be expected.
However, both polyolefin resins and rubbers are poor in adhesiveness, and hence it is difficult to allow polyolefin resins and rubbers to adhere to each other by using adhesives or adhesive tapes; thus, powerful techniques for producing composite materials durable to use in a harsh environment subject to strong vibration or stress are regarded as necessary.
Similarly to the production of composite materials from polyolefin resins and rubbers, also in the adhesives for footwear used in production and repair of footwear, excellent adhesiveness to materials such as polyolefin resins and rubbers is required. Excellent adhesiveness to resins other than polyolefin resins, leather materials and materials constituting woven fabrics/knitted fabrics is also required.
For example, Patent Literature 1 and Patent Literature 2 have proposed adhesive compositions obtained by combining chloroprene rubbers and tackifying agents, as adhesives excellent in adhesiveness to rubber.
However, these adhesives are insufficient in adhesiveness; in particular, these adhesives are poor in the adhesiveness to polyolefin resin, and cannot allow polyolefin resin and rubber to sufficiently adhere to each other.
The adhesive proposed in Patent Literature 1 is a solvent-based adhesive, and is not desirable from the viewpoint of the health of the users and the environmental conservation. On the other hand, the adhesive proposed in Patent Literature 2 is an aqueous adhesive; however, this aqueous adhesive sometimes causes cissing at the time of application thereof depending on the type of the substrate, and sometimes causes aggregation of the polychloroprene latex when a small amount of an organic solvent such as an alcohol is added in order to improve the wettability.
As for the adhesive composition prepared by combining a chloroprene rubber and a tackifying agent, the coating film (adhesive layer) obtained by applying and drying this composition has stickiness or tackiness on the surface thereof even at room temperature. Accordingly, for example, when the adhesive is applied to a sheet-like adherend and dried, and then the adherend is intended to be wound up to a roll to be stored at room temperature, the adherend sometimes cannot be wound up or wound off, and sometimes has dust adhering thereto to degrade the adhesive strength at the time of adhesion. As the method for suppressing the stickiness or the tackiness of the surface of the coating film, a method has been known in which a wax or an inorganic particle is added to the adhesive composition to impart slippage to the surface of the coating film. However, adoption of such a method sacrifices the adhesiveness, and hence it has been difficult to ensure the compatibility between the adhesiveness and the slippage.
A laminate prepared by adhesion with an adhesive composition prepared by combining a chloroprene rubber and a tackifying agent tends to be degraded in the adhesive strength thereof when repeatedly bent in a subfreezing environment.
Patent Literature 3 proposes an aqueous primer made adaptable to polyolefin resin by combining a chloroprene rubber and a tackifying agent with an ethylene-vinyl acetate resin, but does not present the performance of the aqueous primer as an adhesive.
Most of the adhesives for footwear are solvent-based adhesives, and involves issues from the viewpoint of, for example, the environmental conservation as described above; accordingly, for example, Patent Literature 4 and Patent Literature 5 have proposed aqueous adhesives for footwear.
However, the adhesive described in Patent Literature 4 requires a preliminary treatment of the surface of the adherend with a plasma treatment or a solvent-based primer, and hence disadvantageously complicates the involved production step to increase the cost. Moreover, as a result of the use of a solvent-based primer, the issues such as environmental conservation cannot yet be solved.
The adhesive described in Patent Literature 5 limits adherable materials to specific materials; for example, the adhesiveness to polyolefin resin has not yet been investigated.
As described above, no adhesive capable of allowing a polyolefin resin and a rubber to sufficiently adhere to each other has yet been obtained.
The aqueous composition prepared by combining a chloroprene rubber and a tackifying agent undergoes aggregation due to the addition of an organic solvent such as an alcohol, and hence does not allow the wettability thereof.
The adhesive composition prepared by combining a chloroprene rubber and a tackifying agent disadvantageously results in too strong stickiness and tackiness of the surface of the coating film at around room temperature. A laminate obtained by adhesion with this adhesive composition tends to be degraded in the adhesive strength when repeatedly bent in a subfreezing environment, and hence is disadvantageous in cold regions in winter.
Among aqueous adhesives for footwear, no adhesives have yet been found which ensure the compatibility between the sufficient adhesiveness and the cost and environmental conservation.